Apple has issued official guidance - and its chief executive Steve Jobs has issued unofficial guidance - on how to avoid the widely-noted reception problems with the new iPhone 4 when held from below: don't hold it that way.

The cause has been narrowed down to a circuit being formed when the skin bridges the gap between the left and bottom antennas that form the phone's outer stainless steel bezel: depending on the conductance of the skin, it can make the phone reception drop off dramatically.

Emailed by the website Ars Technica about the problem, Jobs responded in his normal terse manner: "Just avoid holding it in that way," he wrote - a response from Jobs that was received by a number of gadget review sites and customers in the US.

Or, in Apple's official statement - which is only being passed out to news organisations which ask for a statement on the problem, rather than sent out (as happens with iPad or iPhone sales figures): "Gripping any phone will result in some attenuation of its antenna performance with certain places being worse than others depending on the placement of the antennas. This is a fact of life for every wireless phone. If you ever experience this on your Phone 4, avoid gripping it in the lower left corner in a way that covers both sides of the black strip in the metal band, or simply use one of many available cases."

The problem emerged within hours of the first iPhone 4 devices being delivered to customers in the US, with dozens posting videos to YouTube showing mobile reception dropping off dramatically when they picked up the phone. Some users wondered why Apple had not spotted the problem during its testing of the iPhone 4 before its launch, and wondered whether the £25 "bumpers" that Apple sells to go around the casing - and protect the metal antennas - was an implicit acknowledgement of the problem. Apple had no comment on Friday on whether it had discovered the problem during testing, or only after the phone went on sale.

Spencer Web, an antenna engineer with Antennasys, notes in a blog post that antennas for mobile phones are generally placed at the bottom of the phone, in order to keep the radiative parts of the phone as far away from the user's head as possible. In the US the Federal Communications Commission measures the output of a phone, and the amount of non-ionising radiation given off, in specific tests - but Web says that those would not have picked up the problem: "when the FCC tests are run, the head is required to be in the vicinity of the phone. But, the hand is not!"

He added: "The antenna structure for the cell phone is still down at the bottom (I won't address the WiFi nor GPS antennas in this blog entry). The iPhone 4 has two symmetrical slots in the stainless frame. If you short these slots, or cover them with your hand, the antenna performance will suffer (see this video I found on YouTube). There is no way around this, it's a design compromise that is forced by the requirements of the FCC, AT&T, Apple's marketing department and Apple's industrial designers, to name a few."

There is some speculation that the problem only occurs on some phones because the antennas are usually covered with a clear finishing solution; if the finish did not cover them properly the phone might be more liable to the problem.

Meanwhile iPhone 4 users are developing their own solutions - which range from the use of duct or masking tape on the bottom edge, painting clear nail varnish over the gap, buying cases to cover the sides of the phone, or - the most expensive option - buying Apple's "bumpers" which cover the edge of the phone.

Meanwhile another problem has appeared on some of the new models: yellow discoloration under the screen. According to one suggestion, this is due to a chemical used in the fixing process that joins the touchscreen to the glass above it and will fade in a few days. That remains to be seen.